Thermal bus or junction for the removal of heat from electronic components

ABSTRACT

A thermal bus enables the use of multiple separate heat pipe assemblies instead of using a single heat pipe assembly spanning the distance from heat source to cold plate. The use of a thermal bus can decrease the orientation effects as well as decrease the travel length of any single heat pipe assembly. In addition, the use of multiple heat pipe assemblies enables each individual heat pipe assembly to be optimized to meet localized heat transfer characteristics between each heat source, the thermal bus, and the cold plate. Such optimization can include the use of differently sized heat pipes, wick structures within the heat pipe, and working fluid used within the heat pipe. The thermal bus provides an intermediate thermal transfer from one heat pipe assembly serially coupled to another heat pipe assembly, thereby enabling multiple serially coupled heat pipe assemblies to transfer heat from a given heat source to the cold plate at the edge of the electronics board.

RELATED APPLICATIONS

This application claims priority of U.S. provisional application, Ser.No. 61/068,891, filed Mar. 10, 2008, and entitled “Fan Tray forSupplemental Air Flow”, by these same inventors. This applicationincorporates U.S. provisional application, Ser. No. 61/068,891 in itsentirety by reference.

FIELD OF THE INVENTION

The invention relates to a method of and apparatus for cooling a heatproducing device in general, and specifically, to a method of andapparatus for cooling electronic components using fluid-based coolingsystems.

BACKGROUND OF THE INVENTION

Cooling of high performance integrated circuits with high heatdissipation is presenting significant challenge in the electronicscooling arena. Electronics servers, such as blade servers and rackservers, are being used in increasing numbers due to the higherprocessor performance per unit volume one can achieve. However, the highdensity of integrated circuits also leads to high thermal density, whichis beyond the capability of conventional air-cooling methods.

A particular problem with cooling integrated circuits on electronicsservers is that multiple electronics servers are typically mounted inclose quarters within a server chassis. In such configurations,electronics servers are separated by a limited amount of space, therebyreducing the dimensions within which to provide an adequate coolingsolution. Often electronics server stacks within a single server chassisare cooled with one or more fans, one or more heat sinks, or acombination of both. Using this configuration, the integrated circuitson each electronics server are cooled using the heat sink and the largefan that blows air over the heat sink, or simply by blowing air directlyover the electronics servers. However, considering the limited freespace surrounding the stacked electronics servers within the serverchassis, the amount of air available for cooling the integrated circuitsis limited.

Many conventional electronic cooling solutions use heat pipe assembliesto remove and reject heat from an electronic component and to thesurrounding air flow or rejected to another heat sink, such as a liquidcold plate. Due to the nature of heat pipes, the thermal performance ishighly sensitive to orientation and length between the electroniccomponent and the heat sink. Heat pipes are generally configured to relyon wicking structures and are aided by gravitational effects. Acondenser portion is positioned at a top of the heat pipe assembly andan evaporation portion is positioned at a bottom, which is thermallycoupled to a heat source such as an electronic component. Heat from theelectronics component evaporates liquid in the evaporation portion ofthe heat pipe assembly. This vapor rises from the bottom to the top ofthe heat pipe assembly, where the vapor condenses to the liquid. Theliquid drains from the top to the bottom of the heat pipe assembly,thereby forming a loop. The greater the distance between the evaporationend and the condensation end of the heat pipe assembly, the moresensitive the heat pipe performance is to the heat pipe orientation. Forblade server applications, this is a greater issue because of the highaspect ratio of the blade itself. These thin blades are normally placeon one edge on the bottom of the rack chassis.

FIG. 1 illustrates an exemplary block diagram of a electronics boardturned on edge, such as in the case of a mounted blade server. Theelectronics board 10 includes multiple electronic components, such ascentral processing unit (CPU) 12, CPU 14, memory device 16, and memorydevice 18. Each of the electronic components generates heat to beremoved. In one manifestation of a heat removal design, air flow isgenerated across the electronic components. Heat from the electroniccomponents is transferred to the crossing air flow. In anothermanifestation of a heat removal design, the heat is transferred from theelectronic components to a liquid cold plate. The cold plate, which canbe attached to an edge of the electronics board, serves as a heat sinkfor the heat generated by the electronic components. Heat pipeassemblies couple the electronic components to the liquid cold plate sothat heat is transferred from the electronic components to the edge(s)of the electronics board. Unfortunately, the reduced performance of theheat pipe assembly due to the orientation problem associated with heatpipe assemblies, allows only the top edge of the electronics board to beused as a viable heat sink. For the electronic components nearer thebottom edge of the electronics board, the heat must be transferred tothe top edge of the electronics board. Increasing the length of the heatpipe assemblies drastically reduces the thermal performance of the heatpipe assemblies.

The physical layout of an electronics board may require bends and turnsin the heat pipe assemblies, as well as increasing the length, to workaround physical obstacles on the electronics board. In some cases, thephysical layout may prohibit positioning the cold plate at the closestposition to the electronic components, further lengthening the heat pipeassemblies. Bends, turns, and increased length all decrease the thermalperformance of a heat pipe assembly.

SUMMARY OF THE INVENTION

Embodiments of the present invention are directed to a thermal bus, orjunction, that enables the use of multiple separate heat pipe assembliesinstead of using a single heat pipe assembly spanning the distance fromheat source to cold plate. The use of a thermal bus, which may be a heatpipe assembly or a liquid loop, can decrease the orientation effects aswell as decrease the travel length of any single heat pipe assembly. Inaddition, the use of multiple heat pipe assemblies enables eachindividual heat pipe assembly to be optimized to meet localized heattransfer characteristics between each heat source, the thermal bus, andthe cold plate. Such optimization can include, but is not limited to,the use of differently sized heat pipes, wick structures within the heatpipe, and working fluid used within the heat pipe. The thermal busprovides an intermediate thermal transfer from one heat pipe assemblyserially coupled to another heat pipe assembly, thereby enablingmultiple serially coupled heat pipe assemblies to transfer heat from agiven heat source to the cold plate at the edge of the electronicsboard.

Another advantage of using a thermal bus is the elimination of sharpbending angles of the heat pipe assemblies. The heat removal capacity ofa heat pipe assembly can be adversely affected by bends in the heat pipestructure. The use of thermal bus eliminates or minimizes the number andseverity of the bends. This technique can be extended to the use ofmultiple thermal buses to eliminate bends in a single heat pipeconfiguration from heat source to cold plate.

Besides taking heat directed from the electronic components, an air-finheat pipe assembly can be used to remove heat from the air flow as well.This air-fin heat pipe assembly can also be attached to the thermal bus.Subsequently, the heat taken from the air flow via the air-fin heat pipeassembly can then be transferred to the cold plate.

The air-fin heat pipe assemblies can be placed at the upstream sideand/or the downstream side of the electronics board. When the air-finheat pipe assemblies are placed at the downstream side, the heatgenerated from the electronic components and expelled into the air flowcan be absorbed by the air-fin heat pipe assemblies and rejected to theliquid cold plate via the thermal bus. Alternatively, the air-fin heatpipe can be placed at the upstream side, the air initially crossing theelectronics board is cooled. This becomes important should the inlet airtemperature rise above the optimal value. In conventional coolingsystems, a higher air temperature necessitates an increase in air flowto increase the cooling capacity. This increase in air flow requiresincreased power to the cooling fans that generate the air flow.Therefore, with increased inlet air temperature, the overall energyefficiency of such a cooling system is decreased. However, with theair-fin heat pipe assemblies placed at the upstream entrance of theelectronics board, the air-fin heat pipe assemblies serve to pre-coolthe air temperature. In this case, the increase in air flow is notneeded.

The air-fin heat pipe assembly design can be used in conjunction withthe heat pipe assemblies coupled to the heat sources to form a hybriddesign, which incorporates both the heat removal directly off electroniccomponents as well as from the air flow. This hybrid configurationallows for the direct removal of heat from larger heat emittingcomponents as well as the indirect heat removal from the aggregate ofthe smaller heat emitting components.

Other features and advantages of the present invention will becomeapparent after reviewing the detailed description of the embodiments setforth below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary block diagram of a electronics boardturned on edge.

FIG. 2 illustrates an exemplary block diagram of a cooling systemincluding a thermal bus according to an embodiment of the presentinvention.

FIG. 3A illustrates a top down view of a first embodiment of the thermalbus.

FIG. 3B illustrates a side view of the first embodiment of FIG. 3A.

FIG. 3C illustrates a side view of a second embodiment of the thermalbus.

FIG. 3D illustrates a side view of a third embodiment of the thermalbus.

FIG. 4 illustrates an exemplary block diagram of a cooling systemincluding multiple thermal buses according to an embodiment of thepresent invention.

FIG. 5A illustrates an exemplary block diagram of a cooling systemincluding an air-fin heat pipe assembly and a thermal bus according toan embodiment of the present invention.

FIG. 5B illustrates an exemplary block diagram of a cooling systemincluding an air-fin heat pipe assembly and a thermal bus according toanother embodiment of the present invention.

FIG. 6 illustrates an exemplary block diagram of a cooling systemincluding a hybrid configuration according to an embodiment of thepresent invention.

The present invention is described relative to the several views of thedrawings. Where appropriate and only where identical elements aredisclosed and shown in more than one drawing, the same reference numeralwill be used to represent such identical elements.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Reference will now be made in detail to the embodiments of the coolingsystem of the invention, examples of which are illustrated in theaccompanying drawings. While the invention will be described inconjunction with the embodiments below, it will be understood that theyare not intended to limit the invention to these embodiments andexamples. On the contrary, the invention is intended to coveralternatives, modifications and equivalents, which may be includedwithin the spirit and scope of the invention as defined by the appendedclaims. Furthermore, in the following detailed description of thepresent invention, numerous specific details are set forth in order tomore fully illustrate the present invention. However, it will beapparent to one of ordinary skill in the prior art that the presentinvention may be practiced without these specific details. In otherinstances, well-known methods and procedures, components and processeshaven not been described in detail so as not to unnecessarily obscureaspects of the present invention.

Embodiments of the present invention are directed to a cooling systemthat transfers heat generated by one or more heat generating devices onan electronics board. The cooling system described herein can be appliedto any electronics board including, but not limited to, a motherboard, agraphics card, or any electronics sub-system that is mounted to abackplane, including, but not limited to, a blade server and a rackserver, herein referred to collectively as an electronics board. Anelectronics board can be coupled to a backplane or mid-plane within aserver or computer chassis. Embodiments of the cooling system aredescribed below in the context of one or more heat generating devicescoupled to an electronics server. It is understood that these conceptscan be expanded to include the cooling of one or more heat generatingdevices coupled to any type of electronics board. As used herein,references to a heat source, a heat generating source, a heat generatingdevice, and the like, as well as specific references to exemplary heatgenerating devices such as an integrated circuit, an integratedmicroprocessor circuit, and a semiconductor heat source, are usedinterchangeably and refer in general to any apparatus or source capableof generating heat.

Embodiments of the cooling system includes a thermal bus, or junction,that thermally couples separate heat pipe assemblies. As used herein, aheat pipe assembly is a device that is made of a thermally conductivematerial with a fluid sealed within the heat pipe assembly interior. Theheat pipe assembly includes an evaporation end and a condensation end.Heat is transferred to the heat pipe assembly at the evaporation end,where the liquid evaporates. The vapor moves from the evaporation end tothe condensation end, where the vapor releases heat thereby condensingthe vapor into liquid. The liquid moves from the condensation end backto the evaporation end. The thermal bus provides a thermal interfacebetween the condensation end of a first heat pipe assembly and theevaporation end of a second heat pipe assembly. As a first fluid in thefirst heat pipe assembly condenses from vapor to liquid at thecondensation end, heat is released. This released heat is thermallytransferred to the evaporation end of the second heat pipe assembly suchthat a second fluid in the second heat pipe assembly evaporates from aliquid to a vapor. The evaporated second fluid then moves to thecondensation end of the second heat pipe assembly.

FIG. 2 illustrates an exemplary block diagram of a cooling systemincluding a thermal bus according to an embodiment of the presentinvention. An electronics board 100 includes multiple electroniccomponents, such as a CPU 102 and a CPU 104. Each of the electroniccomponents generates heat to be removed. Although only two electroniccomponents are shown in the exemplary configuration of FIG. 2, it isunderstood that more, or less, than two electronic components can becoupled to the electronics board 100. As also shown in FIG. 2, theelectronics board 100 is positioned on edge, as in a blade server. Inthis configuration, an electronics board edge 101 is a top edge and anelectronics board 103 is a bottom edge. This configuration is forexemplary purposes only. In an alternative embodiment, the electronicsboard 100 is positioned horizontally, such as component-side up orcomponent-side down.

A heat pipe assembly 106 includes an evaporation end coupled to anelectronics component 102, such as a processor, and a condensation endcoupled to a thermal bus 10. In some embodiments, the evaporation end ofthe heat pipe assembly includes a flat surface to be coupled directly orindirectly via a thermal interface material to the electronic component102. In other embodiments, the evaporation end of the heat pipe assemblyis fitted within a thermally conductive block, which in turn isthermally coupled to the electronic component 102. A heat pipe assembly104 includes an evaporation end coupled to an electronics component 104,such as a processor, and a condensation end coupled to the thermal bus110. The evaporation end of the heat pipe assembly 104 can be thermallycoupled to the electronic component 104 in one of the manners describedabove in relation to the evaporation end of the heat pipe assembly 106.A heat pipe assembly 112 includes an evaporation end coupled to thethermal bus 110 and a condensation end coupled to a cold plate 120. Thecondensation end of the heat pipe assembly 112 can be thermally coupledto the cold plate 120 in one of the manners described above in relationto the evaporation end of the heat pipe assembly 106. In this exemplaryconfiguration, the cold plate 120 is coupled to the top edge 101 of theelectronics board 100 to allow gravity to improve the efficiency of theheat pipe assemblies. However, this is not a requirement, and the coldplate can be positioned on any of the electronic board edges, with theheat pipe assemblies and thermal bus appropriately positioned to providea condensation end of a heat pipe assembly at a location of the coldplate.

In some embodiments, the cold plate 120 is a fluid-based cold plate. Thecold plate is made of a thermally conductive material configured withfluid channels to allow fluid to pass through. Heat is transferred fromthe heat pipe assembly 112 to the cold plate and to the fluid flowingthrough the channels of the cold plate. In some embodiments, thefluid-based cold plate is coupled to a cooling loop. FIG. 7 illustratesthe electronics board 100 coupled to an external fluid-based coolingloop. The cooling loop includes the fluid-based cold plate 120, a heatrejector 140, and a pump 142. The fluid cold plate 120 is coupled to theheat rejector 140 and the pump 142 via fluid lines 144. In otherembodiments, the cold plate 120 can be any heat exchanging device thattransfers heat from the heat pipe assembly 112.

The thermal bus 110 is configured as an intermediate heat exchangerbetween two heat pipe assemblies. In some embodiments, the thermal bus110 is formed by stacking the evaporating end of one heat pipe assemblyon top of the condensation end of another heat pipe assembly, orstacking the condensation end of one heat pipe assembly on top of theevaporating end of another heat pipe assembly. In some embodiments, theends of each heat pipe assembly that form a thermal bus include a flatsurface to thermally couple with each other, either directly orindirectly via a thermal interface material. Although the ends of eachheat pipe assembly 106, 108, and 112 are described above as beingcylindrical or flattened, the shape of the ends can be alternativelyconfigured to mate with each other to form a larger contact surface areafor thermal interface. Similarly, the ends of the heat pipe assemblies106, 108, and 112 can be configured to mate with the corresponding coldplate 120 (FIG. 2), electronic component 102 (FIG. 2), or electroniccomponent 104 (FIG. 2).

In some embodiments, each end of each heat pipe assembly that forms thethermal bus is fitted within a thermally conductive block, and theblocks are thermally coupled to each other. FIG. 3A illustrates a topdown view of a first embodiment of the thermal bus. FIG. 3B illustratesa side view of the first embodiment of FIG. 3A. The evaporation end ofthe heat pipe assembly 112 is positioned in a hole of a thermallyconductive block 111. The condensation end of the heat pipe assembly 106is positioned in a hole of a thermally conductive block 107. Thecondensation end of the heat pipe assembly 108 is positioned in a holeof a thermally conductive block 109. The block 109 and the block 108 areeach thermally coupled to the block 111. In some embodiments, each ofthe blocks 107, 109, and 111 have a flat surface, which are mated toeach other. In some embodiments, a thermal interface material ispositioned between the block 109 and the block 111, and a thermalinterface material is positioned between the block 107 and the block111. The block 111 is coupled to the blocks 107 and 109 by anyconventional securing means including, but not limited to, adhesive,bonding material, solder, mechanical clamp, screw, or bolt, that enablesand forms a thermal interface between the evaporation end of the heatpipe assembly 112 and the condensation ends of each of the heat pipeassemblies 106 and 108. In some embodiments, the block and ends of theheat pipe assemblies are press fit together to ensure thermalconductivity between the heat pipe assemblies and the block. In someembodiments, an additional thermal interface material is positionedbetween the outer surface of the ends of the heat pipe assemblies andthe surface in the holes of the block.

In some embodiments, the evaporating end of each of the heat pipeassemblies 106 and 108 are fitted within a common thermally conductiveblock. This common block is then thermally coupled to the block 111.

As shown in FIGS. 3A and 3B, the appropriate ends of the heat pipeassemblies 106, 108, 112 are fully embedded in the corresponding blocks108, 109, 111, respectively. Alternatively, the ends and the blocks canbe configured such that an end is only partially embedded in a blocksuch that a portion of the end is exposed. This exposed portion can thenbe thermally coupled to an exposed portion of a partially embedded endof another heat pipe assembly. FIG. 3C illustrates a side view of asecond embodiment of the thermal bus. The thermal bus of FIG. 3C issimilar to the thermal bus of FIG. 3B except that the ends of thethermal bus assemblies are not completely embedded in the blocks.Specifically, the evaporation end of a heat pipe assembly 112′ ispartially embedded in a thermally conductive block 111′. A condensationend of a heat pipe assembly 106′ is partially embedded in a thermallyconductive block 107′, and a condensation end of a heat pipe assembly108′ is partially embedded in a thermally conductive block 109′. Anexposed portion of the condensation end of the heat pipe assembly 106′is thermally coupled to an exposed portion of the evaporation end of theheat pipe assembly 112′, either directly or indirectly via a thermalinterface material. An exposed portion of the condensation end of theheat pipe assembly 108′ is thermally coupled to an exposed portion ofthe evaporation end of the heat pipe assembly 112′, either directly orindirectly via a thermal interface material. In some embodiments, theevaporation end of the heat pipe assembly 112′ and the condensation endof the heat pipe assemblies 106′ and 108′ are not cylindrical. In somecases, as is shown in FIG. 3C, these ends are compressed together toform two opposing flat surfaces connected by two radial surfaces. One ofthe flat surfaces is mated to a flat surface of another end.

In some embodiments, the thermal bus 110 includes a single thermallyconductive block including holes into which the ends of the heat pipeassemblies are positioned. FIG. 3D illustrates a side view of a thirdembodiment of the thermal bus. A thermally conductive block 130 includesa hole 136, a hole 132, and a hole 134. The evaporation end of the heatpipe assembly 112 is positioned in the hole 136. The condensation end ofthe heat pipe assembly 108 is positioned in the hole 132. Thecondensation end of the heat pipe assembly is positioned in the hole106. The block 130 functions as a thermal interface material between theheat pipe assemblies 106, 108, and 112. In some embodiments, the blockand ends of the heat pipe assemblies are press fit together to ensurethermal conductivity between the heat pipe assemblies and the block. Insome embodiments, an additional thermal interface material is positionedbetween the outer surface of the ends of the heat pipe assemblies andthe surface in the holes of the block.

In some embodiments, the heat pipe assemblies are coupled to thethermally conductive block such that the evaporating end(s) ispositioned above the condensation end(s), as shown in FIG. 3A-3D. Inother embodiments, the heat pipe assemblies are coupled to the thermallyconductive block such that the condensation end(s) is positioned abovethe evaporating end(s). In still other embodiments, there is nopreference as to the position of the evaporation end or the condensationend in the block as long as there is sufficient thermal coupling betweenthe evaporation end the conduction end.

Referring back to FIG. 2, a single heat pipe assembly is shown betweeneach electronic component and the thermal bus, or between the thermalbus and the cold plate. In some embodiments, multiple heat pipeassemblies can be coupled between an electronic component and thethermal bus and/or between the thermal bus and the cold plate. Theconfiguration in FIG. 2 also shows two electronic components coupled tothe thermal bus. It is understood that more or less than two electroniccomponents can be coupled to the thermal bus. It is also understood thatnot every electronic component on the electronics board 100 need becoupled to the thermal bus. In some embodiments, only select electroniccomponents are coupled to the thermal bus. The configuration of thethermal bus is application-specific to accommodate theapplication-specific number of heat pipe assemblies that are thermallycoupled. Accordingly, where the thermal bus is configured using athermally conductive block, as in FIG. 3C, the number of holes in theblock is also application-specific.

The use of a thermal bus can decrease the orientation effects as well asdecrease the travel length of any single heat pipe assembly. Forexample, a single heat pipe assembly that couples the electroniccomponent 104 to the cold plate 120 has a greater length than either theheat pipe assembly 108 or the heat pipe assembly 112. In addition, eachindividual heat pipe assembly coupled to the thermal bus can be designedto optimize the local heat transfer characteristics corresponding tothat particular heat pipe assembly. For example, the heat pipe assembly108 can be optimized to the heat transfer characteristics in theelectronic component 104/thermal bus 110 domain, and the heat pipeassembly 112 can be optimized to the heat transfer characteristics inthe thermal bus 110/cold plate 120 domain. Such heat pipe assemblyoptimization can include, but is not limited to, the use of differentlysized heat pipes, wick structures, and working fluid.

If the heat transfer path between a specific electronic component andthe cold plate is considered as a series of thermally coupled heat pipeassemblies and thermal bus(es), then each heat pipe assembly in theseries can be independently configured. For example, the heat pipeassembly 106 can have a different configuration than the heat pipeassembly 112. Different heat pipe assemblies can have different interiorwicking structures, different wicking mechanisms such as groove or foam,different types of fluids, and different physical dimensions. Anadvantage of using different heat pipe configurations is to optimizeheat pipe performance for each heat pipe assembly. Individual heat pipeassemblies are optimized to work in specific temperature ranges. Whenthe temperature extends beyond the designed temperature range,performance decreases. So one heat pipe assembly can be optimizeddifferently than another heat pipe assembly to match the respectiveoperating temperature ranges of each. As each individual heat pipeassembly has a limited operating temperature range, use of multiple heatpipe assemblies, as enabled by the thermal bus, extends the overalltemperature range to which the cooling system can be applied. As opposedto using a single heat pipe assembly with a single fluid type andstructure, the single heat pipe assembly is segmented into multiple heatpipe assemblies, each segment corresponding to a different temperaturerange and each segmented heat pipe assembly optimized to thecorresponding temperature range.

By way of example, consider the heat transfer path between theelectronic component 102 and the cold plate 120, which includes the heatpipe assembly 106, the thermal bus 110, and the heat pipe assembly 112.For hotter temperature ranges, such as at the electronics component 102,water is a good fluid choice for use in a heat pipe assembly. However,water does not behave as well in extreme cold, such as at the interfaceat the cold plate 120. If a single heat pipe assembly is used betweenthe electronic component 102 and the cold plate 120, this is anundesirable temperature range for water due to the low temperature atthe cold plate 120. However, alcohol does behave well at lowertemperatures and is a good design choice for a heat pipe assemblycoupled to the cold plate 120. But alcohol does not behave well athotter temperatures, such as at the electronics component 102. If asingle heat pipe assembly is used between the electronic component 102and the cold plate 120, this is an undesirable temperature range foralcohol due to the high temperature at the electronics component 102. Byusing the two heat pipe assemblies 106 and 112, the heat pipe assembly106 can be designed using water, and the heat pipe assembly 112 can bedesigned using alcohol. This improves thermal performance of the overallheat removal process. It is understood that this is a simplifiedexample, and that other factors other than the fluid type can be takeninto consideration in determining the respective configurations of theheat pipe assemblies.

Selecting the proper fluid type is not the only challenge for aconventional single heat pipe assembly configuration. An electroniccomponent is often conventionally cooled using a single heat pipeassembly that includes an evaporation portion coupled to the heatgenerating electronic component and a condensation portion having anair-fin structure. Air moving across the air-fin structure removes heatcausing condensation of the fluid within the heat pipe assembly. In anexemplary application, an integrated circuit operates at an approximatetemperature of 60 degrees Celsius. Ambient air temperature beforeheating is approximately 25-30 degrees Celsius, which results in anoperating temperature range of approximately 30-35 degrees Celsius.However, if the heat removal process is improved using a cold plateinstead of air cooling, the operating temperature range increases toapproximately 45 degrees Celsius for a cold plate operating at atemperature of approximately 15 degrees Celsius. This increase inoperating temperature range is enough to negatively impact the overallthermal performance of the single heat pipe assembly.

In an exemplary application of the multiple heat pipe assemblies andthermal bus of the present invention, an operating temperature of thethermal bus is approximately 30-35 degree Celsius. With the approximateoperating temperature of the electronics component again being 60degrees Celsius, the operating temperature range of the heat pipeassembly 106 is approximately 30-35 degrees Celsius, which is aneffective temperature range and an effective absolute temperature forwater. Within the operating temperature range of the cold plate 120again being approximately 15 degrees Celsius, the operating temperaturerange of the heat pipe assembly 112 is approximately 15-20 degreesCelsius, which is also an effective temperature range and an effectiveabsolute temperature for alcohol. The operating temperature of thethermal bus 110 is sufficiently low to condense the water in the heatpipe assembly 106, and sufficiently high to evaporate the alcohol in theheat pipe assembly 112. It is again understood that this is a simplifiedexample, and that other factors other than the fluid type can be takeninto consideration in determining the respective configurations of theheat pipe assemblies.

In general, the portion of the heat pipe assembly 106 coupled at thethermal bus 110 must provide condensation of the fluid used in the heatpipe assembly 106, and the portion of the heat pipe assembly 112 coupledat the thermal bus 110 must provide evaporation of the fluid used in theheat pipe assembly 112. These condition can be met by appropriateconfiguration of the heat pipe assemblies using different fluid types,different conditions such as pressure within the heat pipe, anddifferent heat pipe physical structures.

The heat removal capacity of the heat pipe assembly can be adverselyaffected by bends in the heat pipe assembly structure. The use of athermal bus eliminates or minimizes the number and severity of thebends. The cooling system of FIG. 2 is described in terms of a singlethermal bus coupling two heat pipe assemblies. These concepts can beextended to couple additional thermal buses and heat pipe assemblies inseries. The use of multiple thermal buses can be used to eliminatemultiple bends, to accommodate restricted space due to other componentson or near the electronics board 100, and to accommodate differentlocations of the cold plate 120 relative to the electronics board 100.The use of multiple thermal buses can also be used to further optimizethe local heat transfer characteristics, as described above.

FIG. 4 illustrates an exemplary block diagram of a cooling systemincluding multiple thermal buses according to an embodiment of thepresent invention. The cooling system of FIG. 4 is similar to thecooling system of FIG. 2 with the addition of a second thermal bus 130.Heat pipe assembly 114 functions similarly as the heat pipe assembly 112except that the condensation end of the heat pipe assembly 114 iscoupled to the thermal bus 130, instead of to the cold plate 120. Anevaporation end of a heat pipe assembly 116 is thermally coupled to thecondensation end of the heat pipe assembly 114 via the thermal bus 130.A condensation end of the heat pipe assembly is thermally coupled to thecold plate 120. The thermal bus 130 functions in a similar manner as thethermal bus 110. The physical configuration of the thermal bus 130 canbe the same as the configuration of the thermal bus 110, such as boththermal buses include a thermally conductive block as in FIG. 3C.Alternatively, the physical configuration of the thermal bus 120 can bedifferent than the thermal bus 110, such as one thermal bus having athermally conductive block and the other thermal bus using the stackingconfiguration as in FIGS. 3A-3B.

The cooling system of FIG. 4 shows two thermal buses. In someembodiments, more than two thermal buses can be used, thermally coupledby one or more additional heat pipe assemblies. In general, the numberof thermal buses and heat pipe assemblies used in the cooling system islimited only by the thermal performance requirements of the overallsystem. Each thermal bus can be independently configured to couple oneor more condensation ends of heat pipe assemblies to one or moreevaporation ends of other heat pipe assemblies. Thermal buses and heatpipe assemblies can be configured in series or in parallel. Heat can betransferred from one heat pipe assembly to multiple other heat pipeassemblies via the thermal bus, or heat can be transferred from multipleheat pipe assemblies to one or more other heat pipe assemblies via thethermal bus. In some embodiments, the heat generated by a specificelectronic component can be transferred to the cold plate usingdedicated heat pipe assemblies and thermal bus(es).

In addition to or alternatively to taking heat directly from theelectronic components, an air-fin heat pipe assembly can be used toremove heat from the air flow crossing the electronics board. Thisair-fin heat pipe assembly can also be attached to a thermal bus.Subsequently, the heat taken from the air flow can then be transferredto the cold plate. In some embodiments, a heat sink is thermally coupledto one or more electronic components. Air flowing over the electronicsboard absorbs heat as it moves across the heat sink(s). This transfersheat from the electronic component to the heat sink to the air.

FIG. 5A illustrates an exemplary block diagram of a cooling systemincluding an air-fin heat pipe assembly and a thermal bus according toan embodiment of the present invention. The cooling system of FIG. 5Afunctions similarly as the cooling system of FIG. 2 except that no heatpipe assembly is positioned on-device, that is there is no heat pipeassembly coupled directly to the electronic components 102 and 104.Relative to the air flow direction across the electronics board 100, anevaporation end of a air-fin heat pipe assembly 206 and an evaporationend of an air-fin heat pipe assembly 208 are positioned downstream ofthe electronic components 102 and 104. In the exemplary configuration ofFIG. 5A, the evaporation ends of the air-fin heat pipe assemblies 206and 208 are positioned at a back end, or air flow exit, of theelectronics board 100. The evaporation end of the air-fin heat pipeassembly 206 and the evaporation end of the air-fin heat pipe assembly208 are each fitted with fins. A condensation end of the air-fin heatpipe assembly 206 is thermally coupled to the thermal bus 210, and acondensation end of the air-fin heat pipe-assembly 208 is thermallycoupled to the thermal bus 210. Heat is transferred to air flowing overthe electronics components 102 and 104 (and any other heat generatingdevices on the electronics board 100), or over heat sinks coupled to theelectronics components 102 and 104. As the heated air moves across thefins on the evaporation ends of the air-fin heat pipe assemblies 206 and208, heat is transferred to the evaporation ends, thereby evaporatingthe fluid within. In this manner, the temperature of the air exiting theelectronics board 100 is decreased.

In an alternative configuration, air-fin heat pipe assemblies arepositioned upstream of the electronics components. FIG. 5B illustratesan exemplary block diagram of a cooling system including an air-fin heatpipe assembly and a thermal bus according to another embodiment of thepresent invention. The cooling system of FIG. 5B functions similarly asthe cooling system of FIG. 5A except that the evaporation ends of theair-fin heat pipe assembles 306 and 308 are positioned upstream of theelectronic components 102 and 104. In the exemplary configuration ofFIG. 5A, the evaporation ends of the air-fin heat pipe assemblies 306and 308 are positioned at a front end, or air flow entrance, of theelectronics board 100. The evaporating end of the air-fin heat pipeassembly 306 and the evaporating end of the air-fin heat pipe assembly308 are each fitted with fins. A condensation end of the air-fin heatpipe assembly 306 is thermally coupled to the thermal bus 310, and acondensation end of the air-fin heat pipe-assembly 308 is thermallycoupled to the thermal bus 310. Air entering the electronics board iscooled by flowing over the air-fin heat pipe assemblies 306 and 308.Heat is transferred to the cooled air flowing over the electronicscomponents 102 and 104 (and any other heat generating devices on theelectronics board 100), or over heat sinks coupled to the electronicscomponents 102 and 104.

The air-fin heat pipe assemblies can be placed at the entrance, exit, orboth the entrance and exit of the of electronics board. When the air-finheat pipe assemblies are placed at the exit, as shown in FIG. 5A, theheat generated from the electronic components and transferred into theair flow can be absorbed by the air-fin heat pipe assemblies andrejected to the cold plate via the thermal bus. However, if the air-finheat pipe assemblies are placed at the entrance, as shown in FIG. 5B,the entering air is cooled before reaching the electronic components.This configuration is useful should the inlet air temperature rise abovethe optimal value. In most cooling systems, the higher air temperaturenecessitates the increase in air flow to increase the cooling capacity.This increase in air flow requires increase power to the cooling fansthat generate the air flow. In this event, the overall energy efficiencyof the cooling system is decreased. However, placed at the entrance theair-fin heat pipe assemblies serve as pre-coolers or air temperaturecontrollers. The increase in air flow therefore, is reduced if noteliminated.

FIGS. 4A-4B show the air-fin heat pipe assemblies attached to thethermal bus. Alternatively, the air-fm heat pipe assemblies can bedirectly attached to the cold plate 120. If the limitations of theair-fin heat pipe assembly and the physical layout of the electronicsboard prevent the direct connection, a thermal bus can be used.

The air-fin heat pipe assembly design can be combined with the on-deviceheat pipe assembly design as a hybrid design which incorporates bothheat removal directly off electronic components as well as from the airflow. This hybrid configuration allows for the direct removal of heatfrom the larger heat emitting components as well as the indirect heatremoval from the aggregate of the smaller heat emitting components.

FIG. 6 illustrates an exemplary block diagram of a cooling systemincluding a hybrid configuration according to an embodiment of thepresent invention. The exemplary hybrid configuration of FIG. 6 includesthe on-device heat pipe assembly design as shown in FIG. 2 and the aircooling air-fin heat pipe assembly design as shown in FIG. 5A. Air isheated by flowing over electronic components 116 and 118 (and any otherheat generating devices on the electronics board 100), or over heatsinks coupled to the electronics components 116 and 118. Heat istransferred from the heated air to an air-fin heat pipe assembly 406 andan air-fin heat pipe assembly 408, which are each thermally coupled to athermal bus 410. A heat pipe assembly 402 thermally couples theelectronic component 102 to the thermal bus 410, and a heat pipeassembly 404 thermally couples the electronic component 104 to thethermal bus 410. A heat pipe assembly 412 thermally couples the thermalbus 410 to the cold plate 120. Although the hybrid configuration of FIG.6 is shown and described in terms of a downstream configuration, it isunderstood that the upstream configuration of the air cooling air-finheat pipe assembly design of FIG. 5B can be alternatively used or addedto the hybrid configuration.

An attribute of the hybrid design is its ability to self-regulate thecooling capacity of the heat pipe assembly/thermal bus cooling system.Under the ideal conditions, the hybrid heat pipe assembly is designedsuch that the heat directly removed from the electronic components, aswell as the heat indirectly removed via the air flow, is rejected to thecold plate. However, if the heat generated from the electroniccomponents that are directly attached to the heat pipe assemblies exceedthat of the cold plate, or other secondary loop capacity, the air-finheat pipe assemblies that are attached to the thermal bus can act as arejecter of heat to the air flow. The switch from absorbing to rejectingheat is passive in nature and the switching point can be designed intothe original assembly. This scenario can occur if there is a reductionin the cold plate cooling or secondary cooling loop capacity. Therefore,the air-fin heat pipe assemblies attached to the thermal bus can offersome relief or redundancy if the main cooling path is compromised.

In another scenario, the hybrid heat pipe assembly design can be used asan air conditioner for the air exiting the electronics board. If acertain exiting air temperature is desired, the air-fin heat pipeassemblies can either reject or absorb heat to or from the air flow asneeded. If the exiting air is above a desired temperature, the air-finheat pipe assemblies can absorb heat from the air. If the exiting air isbelow the desired temperature, perhaps due to low inlet air temperatureor electronic components operating at low powers, the air-fin heat pipeassemblies can reject heat into the air flow. Again, the control point(desired temperature) can be designed into the original heat pipeassembly/thermal bus cooling system. The need for air temperaturecontrol both at the entrance and exit of the electronics board isdesired for the efficient operation of any rack level, or room levelcooling system design.

The various configurations of the cooling system on the electronicsboard 100 provide adaptable solutions for application-specific thermalrequirements. Such thermal requirements may be dictated by theconfiguration and components on the electronics board itself and/or theconfiguration and components positioned before or after the electronicsboard 100 along the air flow direction. The configuration of the coolingsystem can be determined to account for various input air flowtemperatures and to provide various output air flow temperatures.

It is typically more efficient to transfer heat directly off theelectronic component, such as in FIGS. 2 and 4, than to cool heated air,as in FIGS. 5A-5B. However, there may be limitations such as boardlayout and relative electronic component positions that necessitates oneimplementation versus another. As such, any of the heat transferconcepts and configurations described above can be combined andmanipulated.

The thermal bus can be modular in design. The heat pipe assembly foreach electronic component can be readily attached and detached from thethermal bus. This allows for easy access to the electronic component aswell as easy optimization of the heat pipe solution for each electroniccomponent.

Although heat pipe assemblies typically utilize wicking structures inaddition to gravitational effects, alternative structures can be usedthat rely solely on gravity, for example vapor chambers or thermalsiphons. In blade server applications where the blade server ispositioned on edge, certain configurations may require that the positionof the evaporation end relative to the condensation end works againstgravity. This situation necessitates the use of a heat pipe assemblythat includes wicking structure. In other applications, however, such aswhen the electronics board is horizontally positioned instead ofvertically positioned as in the blade server, the alternative structurescan be used. In an exemplary configuration, a vapor chamber can bepositioned on an electronics component. An evaporating end of the vaporchamber is at the bottom end proximate the electronics component, and acondensing end is at the vertical top end of the vapor chamber. Anevaporating end of a heat pipe assembly is thermally coupled to the topend of the vapor chamber. A thermal bus is formed between the top end ofthe vapor chamber and the evaporation end of the heat pipe assembly.This configuration initially removes heat from the electronic componentin a vertical direction using the vapor chamber. This type of thermalbus provides an additional axis that provides further alternatives fordesigning the overall configuration of the cooling system.

The present invention has been described in terms of specificembodiments incorporating details to facilitate the understanding of theprinciples of construction and operation of the invention. Suchreference herein to specific embodiments and details thereof is notintended to limit the scope of the claims appended hereto. It will beapparent to those skilled in the art that modifications may be made inthe embodiment chosen for illustration without departing from the spiritand scope of the invention.

1. A device to remove heat comprising: a. a first heat pipe assemblyincluding a first end where evaporation of a first fluid occurs, and asecond end where condensation of the first fluid occurs, wherein thefirst end is configured to collect heat; and b. a second heat pipeassembly including a first end where evaporation of a second fluidoccurs, and a second end where condensation of the second fluid occurs,wherein the first end of the second heat pipe assembly is thermallycoupled to the second end of the first heat pipe assembly, therebyforming a thermal bus between the second end of the first heat pipeassembly and the first end of the second heat pipe assembly.
 2. Thedevice of claim 1 wherein the thermal bus is configured to transfer heatgenerated by the condensation of the first fluid at the second end ofthe first heat pipe assembly to the first end of the second heat pipeassembly thereby evaporating the second fluid at the first end of thesecond heat pipe assembly.
 3. The device of claim 1 wherein the heatgenerating device comprises an electronic component.
 4. The device ofclaim 3 wherein the electronic component is coupled to an electronicsboard.
 5. The device of claim 1 further comprising a cold plate coupledto the second end of the second heat pipe assembly.
 6. The device ofclaim 5 wherein the cold plate comprises a fluid-based cold plateincluded in a cooling loop.
 7. The device of claim 5 further comprisingone or more additional second heat pipe assemblies coupled between thefirst heat pipe assembly and the cold plate.
 8. The device of claim 1wherein the first fluid comprises a different type of fluid than thesecond fluid.
 9. The device of claim 1 wherein the first fluid comprisesa same type of fluid as the second fluid.
 10. The device of claim 1wherein a physical structure of the first heat pipe assembly isdifferent than a physical structure of the second heat pipe assembly.11. The device of claim 1 wherein a physical structure of the first heatpipe assembly is the same as a physical structure of the second heatpipe assembly.
 12. The device of claim 1 further comprising one or moreadditional heat pipe assemblies, wherein a first end of each additionalheat pipe assembly is thermally coupled to an additional heat generatingdevice, and a second end of each additional heat pipe assembly isthermally coupled to the first end of the second heat pipe assembly. 13.The device of claim 1 further comprising one or more additional heatpipe assemblies thermally coupled in series with the second heat pipeassembly such that a first end of a first additional heat pipe assemblyis thermally coupled to the second end of the second heat pipe assembly,and a first end of any additional heat pipe assemblies is thermallycoupled to a second end of a previous heat pipe assembly in the series.14. The device of claim 1 wherein an axis of the first heat pipeassembly is at an angle to an axis of the second heat pipe assembly. 15.The device of claim 1 wherein an axis of the first heat pipe assembly isparallel to an axis of the second heat pipe assembly.
 16. The device ofclaim 1 further comprising one or more air-fin heat pipe assemblies,each air-fin heat pipe assembly includes air-fins coupled to a first endof the air-fin heat pipe assembly where evaporation of a third fluidoccurs, and a second end of the air-fin heat pipe assembly wherecondensation of the third fluid occurs, wherein the second end of eachair-fin heat pipe assembly is thermally coupled to the first end of thesecond heat pipe assembly.
 17. The device of claim 1 wherein one or moreof the air-fin heat pipe assemblies are positioned after the heatgenerating device relative to an air flow direction across the heatgenerating device.
 18. The device of claim 1 wherein one or more of theair-fin heat pipe assemblies are positioned before the heat generatingdevice relative to an air flow direction across the heat generatingdevice.
 19. The device of claim 1 wherein one or more of the air-finheat pipe assemblies are positioned before the heat generating deviceand one or more of the air-fin heat pipe assemblies are positioned afterthe heat generating device relative to an air flow direction across theheat generating device.
 20. The device of claim 1 wherein the thermalbus further comprises a thermal interface material positioned betweenthe second end of the first heat pipe assembly and the first end of thesecond heat pipe assembly.
 21. The device of claim 1 wherein the thermalbus comprises a block of thermally conductive material, further whereinthe block includes a plurality of holes, each hole configured to acceptand thermally couple with either the second end of the first heat pipeassembly or the first end of the second heat pipe assembly.
 22. Thedevice of claim 1 further comprising one or more additional first heatpipe assemblies coupled between the heat generating device and thesecond heat pipe assembly.
 23. The device of claim 1 wherein the firstend of the first heat pipe assembly is thermally coupled to a heatgenerating device.
 24. The device of claim 1 wherein the first heat pipeassembly comprises an air-fin heat pipe assembly having air-fins coupledto the first end of the air-fin heat pipe assembly, further wherein thefirst end of the air-fin heat pipe assembly is configured to receiveheat transferred from an air flow contacting the air-fins.